Likely causes of recurring ceiling cracks in newer houses
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Figure A: Typical truss
Warm, moisture-laden air leaking into the attic gets absorbed by cold parts of the truss, expanding the wood and causing the truss to warp slightly. If the drywall can’t move enough to accommodate the warp, it cracks away from the wall.
This problem occurs occasionally
with roof trusses. Trusses are
designed to carry the weight of the roof
on the ends, that is, the outer walls. If a
seasonal crack opens along a wall somewhere
in the middle, one or both truss
ends are rising or the truss is arching. The
first case is a foundation problem. If the
footings under the wall aren’t deep
enough, the freeze/thaw cycles in cold winters could lift the outer walls.
Changing moisture conditions in expansive,
heavy clay soil can also lift the walls,
but that’s probably not the problem unless you’re seeing other wall/
floor gaps or cracks.
The second cause, truss
arching or truss uplift, is a
slight bowing of the entire
truss upward during the winter.
Moisture that escapes
into the attic in cold weather
tends to be absorbed by the cold top chords (Figure A), which
causes them to expand slightly. The
bottom chord doesn’t expand because
it remains buried in the insulation
close to the warm ceiling. The result is
a slight warp, just as a board warps
when one side is damp and the other
remains dry. The warp goes away once
the weather warms and the moisture
content in the wood equalizes.
One solution is to reduce the
humidity in your attic by (1) closing
up air leaks through the ceiling, and
(2) making sure you have good, unobstructed
attic ventilation (soffit and
roof vents).
Another solution: Don’t nail the
ceiling drywall to the trusses within
about 16 in. of any wall. Instead, support
those edges by attaching them to
blocks nailed to the tops of the wall
framing or to special drywall clips
attached to the walls. That frees the
trusses to flex slightly upward without
pulling the drywall corners apart.
